The use of manganese dioxide as an active cathode material (depolarizer) in dry cells is well known. Manganese dioxide for cell use can be derived from natural manganese dioxide ores or it can be electrolytically produced by electrolyzing a manganese sulfate solution as disclosed in the publication titled "Batteries"--Vol. 1, edited by Karl V. Kordesch and published by Marcel Dekker, Inc., New York, 1974. Moreover, manganese dioxide can be produced by suitable chemical processes such as those described in Synthesis, 1976, No. 2 (February) pp. 65-104.
It is known that both electrolytic manganese dioxides (EMD) and some chemically prepared forms of manganese dioxide possess "superactive" sites on their surfaces. The presence of such superactive sites produces adverse consequences regarding the use of these manganese dioxide materials in both aqueous and nonaqueous cells.
The superactive sites on the surface of the manganese dioxide are apparently associated with an abnormally high initial open cell voltage and an undesirable reactivity with other cell components such as the organic solvents in nonaqueous electrolytes and the carbon in cathode mixes of aqueous cell systems.
It is postulated that these superactive sites are acidic in nature. It is believed that these surface acidic groups result in manganese dioxide having an ion-exchange function resulting in protons being dissociated in aqueous electrolyte solutions thus increasing the acidity of the liquid. Although the manganese dioxide used for nonaqueous cells is typically thermally treated at a high temperature and is dehydrated, it still apparently has acidic groups on its surface.
Typically, the nonaqueous electrolytes employed in such cells comprise organic solvents such as ethers and esters, for example propylene carbonate, which solvents are likely to be decomposed by very small amounts of acid or the like, resulting in the production of gas, corrosion of the anode, polymerization of the solvent and other undesirable effects.
Thus, it is desirable to find a process for neutralizing these acidic groups in order to reduce these adverse effects. The neutralization of such acidic groups is not easily accomplished as is apparent from an examination of EMD production processes. EMD is produced in an acid environment and is typically subjected to treatment with bases such as aqueous sodium hydroxide or ammonia prior to use in electrochemical cells. Nevertheless, such base treatments do not neutralize the surface acidic sites on the EMD. Moreover, it is important that treatment of manganese dioxide does not substantially reduce the normal electrochemical activity of the manganese dioxide such that the performance of such MnO.sub.2 in electrochemical cells is adversely affected.
It is therefore an object of this invention to provide a process for the treatment of manganese dioxide which will substantially inactivate the superactive sites located on the surface of such manganese dioxide thereby reducing the initial open circuit voltage of electrochemical cells employing such manganese dioxide, as well as reducing the undesirable degradation reactions of cell components with such manganese dioxide in electrochemical cells.
It is a further object of this invention to provide a process for the inactivation of superactive sites on the surface of manganese dioxide, which process will not substantially reduce the peroxidation value of such manganese dioxide for use in electrochemical cells.